Wide gap semiconductors such as GaN, SiC, or diamond have the feature in which important factors such as dielectric breakdown voltage, electron mobility, and thermal conductivity in a semiconductor device are excellent compared with Si. An HEMT having an AlGaN/GaN hetero structure, which is a type of GaN semiconductor devices, has attracted much attention since the HEMT has the characteristics such as high-frequency performance and low on-resistance originated from its high electron mobility and carrier density. Further, since the HEMT having the AlGaN/GaN hetero structure has the (normally-on) characteristics that the current flows without the application of the gate voltage, a negative voltage needs to be applied to the gate in order to turn off the element, that is, to stop the current.
For this reason, the negative power supply is needed, which causes problems that the number of components of the circuit increases and the wiring of the circuit board becomes complex.
As a circuit that omits the negative power supply, for example, a circuit which includes an oscillator outputting a control signal for turning on or off the HEMT element, a capacitor disposed between a control signal output terminal of the oscillator and a gate of the HEMT element, and a diode disposed between the capacitor and a source of the HEMT element may be supposed. However, this circuit has drawbacks in which the diode and the capacitor having a large capacitance of 10 to 500 times the input capacitance of the FET are needed to be provided outside, in addition to the semiconductor device, and the number of components provided outside the switching element increases. There is also a problem that voltage oscillation (ringing) is generated by parasitic inductances produced in accordance with an increase in the wiring length from the driving circuit of the switching element to the gate of the switching element.